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Maybe not algae, but I'm stumped

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So recently upon switching from API Leaf Zone/CO2 Booster to Flourish+Excel+Trace upon realizing there was prolly a good micronutrient deficit in my tank (and to hop on the bandwagon too, I guess), I noticed that soon after a grey-white powdery stuff was plastered all over the rocks, driftwood and the back of my bamboo shrimp (a fanning shrimp gathers stuff perhaps likened to moss, I guess).

It slowly goes away after a day or so and then appears again when I dose fertilizer, but doesn't fit the description of any algae I've seen in the links provided in the sticky though it doesn't quite look like a fungal thing either. It's truly powdery in appearance, like mildew, and doesn't look cottony or protrude from the stuff it's attached to, at least yet. It isn't some kind of precipitate either, as making a bit of current near it doesn't move the stuff whatsoever, but scrubbing at it does.

I'm leaning towards it being some kind of fungus, myself, but I'd like some opinions before doing anything about it.

I actually think I've figured it out. It IS some kind of fungus but it seemingly takes care of itself over the course of a few days -- and it pops back up basically when there's new nutrients in the tank, so that means not just after adding Flourish/Excel/Trace but when doing a partial water change as well. My ghost shrimp seem to peck at it on top of it, so they're being ever-handy!

I'm fairly certain this fungus began to crop up as a result of using some SeaChem PhosGuard to reduce the phosphates in the tank (were kinda high, and algae was getting a bit annoying as I began to have to scrub the glass daily on top of my snails' work on it), so the sudden lack of competition probably let the fungus flourish. I pretty much quit the PhosGuard regimen (as the phosphate level was low enough to warrant stopping now, anyhow) and intend to try and keep the phosphates a little higher than 0.5ppm, but far lower than it was (I believe I started at >5ppm!) and did a prompt partial water change to up the phosphate a little bit -- the denizens of the tank seemed to like it as my biggest female guppy suddenly had her babies afterward, the bamboo shrimp outright molted immediately and one ghost shrimp is now suddenly VERY berried the next day (can't even tell how many eggs she's got because she's THAT loaded with them). All is well, it seems! :

Uh... turns out the fungus has killed off a good chunk of my biological filter. Today I did my basic water tests (ammonia, nitrate, nitrite and pH) and suddenly everything is all topsy-turvy -- ammonia test read 4.0ppm, nitrate was >20ppm, nitrite was 0.50ppm, and pH had dropped to 6 from around 6.8-7 the previous day.

I'm wholeheartedly surprised nothing had died -- the only sad one was one of the male guppies, but he showed no signs of disease and only rests a lot on the substrate in a quiet corner of the tank but was eating and would chase the ladies when they came poking around his hiding spot. The shrimp and snails seem happier than ever, oddly -- making me think the tests might somehow collectively be off... but for all the tests to swing so widely from the day before? I'm unsure.

Either way, did the obligatory 50% water change, and did a no-no and cleaned off the biowheel in the filter, as where I saw brown algae-ish gunk before, stringy pink/orange-white fungus had completely taken over. Added some biological filter kickstarting stuff (used "Micro-lift Special Blend") to hopefully take over temporarily and hopefully outcompete the fungus and retake the tank for more benign and beneficial to reestablish down the line.

Here's to continued good/dumb luck to accompany my aquarium-newbie semi-educated flailing around like a moron!

Alrighty, a question related to my predicament -- would introducing something like marimo balls to my tank serve as a possible stand-in for the algae I killed, thus letting that fungus run amok? If it can serve as new competition against the fungus, I'd definitely go ahead and grab a couple to try and outcompete this pesky stuff.

And... I guess it can't hurt to post photos of this stuff either. I was kinda avoiding it before both out of stubbornness to mostly-deal with it myself and embarassment about being a newbie playing god with their tank and things going haywire as a result. Getting an ID on this stuff might help combat it.

Also I noticed how it grows since it's gotten a bit worse due to the few water changes I've had to do to just get the water parameters back into sane/workable levels. It maintains its mildewy appearance like I described before, but you can see a distinct kind of a wonky scallop shell-like pattern in the colony's growth. The photos don't really show this pattern too well, though.

The fungus is fading back, but that's likely because I haven't added any fertilizers or done a water change for a day or so -- it often crops back up when doing either, and I had to do several 50% changes to get the nitrates to finally read low enough. I DID go ahead and grab a couple marimo balls and plopped them in the brightest spot in the tank, and I went and added some API Ammo-Lock yesterday, as the water changes prolly are gonna do a number on my plants and what little remains of my biological filter, what without fertilizers. Going to avoid using Excel for now, lest it hampers the marimo. At least that ideally renders the remaining (~1ppm) ammonia in the tank nontoxic to the fish while hopefully encouraging the recovery of the nitrifying bacteria and outcompetition of the fungus.

Also removed the filter from the tank and cleaned it out really well -- the intake tube and impeller housing were completely plastered with the fungus and emitted a lovely sulfrous odor when I cracked it open to clean it all out.

Overall there are no deaths so far even with that pretty MASSIVE ammonia/nitrite/nitrate spike, save perhaps a few guppy fry -- however I have seen at least three of those little guys still alive and kicking and getting more bold as they're slowly approaching the size where they can't be eaten.

you mentioned a sulfurous odor when you opened the filter?
that's a big sign something is wrong. sulfur is produced by anaerobic bacteria,
or bacteria that does not need oxygen to survive. we always want our tanks to be well-oxygenated so something is wrong there. how well is the aeriation for your tank?

I have a rapidly bubbling airstone in there, from an air pump rated for 10 gallons -- this tank is 6. The guppies don't hang around the surface, don't gasp for air and their gills look pink and not red, nor do the snails or shrimp try to hang near the surface at all. I've cleaned out the impeller housing before this as regular maintenance, and this is the first time I've noticed the odor.

I suspect that maybe whatever bacteria are in the bottled nitrifying bacteria I added days ago might have caused it -- it has a sulfrous odor to it straight out of the bottle, so at least some of the bacteria in that mix may be able to function anaerobically. Maybe if the fungus in the filter consumed oxygen, it could have supported the anaerobic bacteria? The water flow isn't slow though, so that may not be the case.

Still fighting the fungus, but it only rears its ugly, mildewy head and blooms back when I do a water change, top off the tank or add Flourish Excel (oddly not when adding Trace or Comprehensive, making me think the stuff really likes dissolved carbons). Fortunately the fungus dies back again within 24 hours afterwards, unlike before where it lingered for days. This is an odd one!

The ammonia's still high (sometimes doubles its concentration in the water within the course of 24 hours) and I'm trying to encourage my biological filter's recovery with bacterial additives while doing water changes as needed and reducing feeding and detritus within the tank. The bacterial additives I've tried are Microbe-Lift "Special Blend" and Nutrafin Cycle.

While using the special blend stuff in other tanks and this one before the fungus problem, a jump in nitrite and nitrate and reduction in ammonia resulted really quickly, whereas in my tank post-fungus, I notice no such trend -- absolutely no change in nitrite or nitrate (both reading 0ppm since the fungus boomed back when I first made this thread) and only water changes seem to dent the ammonia concentration. Now, I realize that the bacteria in those additives may have died off while still in the bottle, but I can't help but wonder if the fungus still has such a monopoly on things in my tank that the bacteria has no chance of working, much less establishing.

(P.S. still no deaths, including the three guppy fry who are now big enough to roam the tank without regard to being eaten by their mom and aunts/uncles!)

That sure looks like rotifers or vorticella on the plants. They attach to plants with springy stems and when disturbed will pull themselves downward. You should see them bouncing up and down. But I've never seen them take over an entire tank like that.